Electromechanical filter



Sept. 15, 1953' M. ANTHONY El AL zfigzjszlz ELECTROMECHANICAL FILTER Filed Dec. 14, 1948 3 Sheets-Sheet 1 IN VEN TORS.

Sept. 15, 1953 M; L. ANTHONY ET AL ELECTROMECHANICAL FILTER 3 Sheets-Sheet 2 Filed Dec. 14, 1948 ffC.

Sept. 15, 1953 M. ANTHONY ET AL 2,652,542

ELECTROMECHANICAL .FILTER Filed Dec. 14, 1948 5 Sheets-Sheet 3 12 1112 115 1? if 12 jgyfoyz J, g i zy 4 57 ammo? Patented Sept. 15, 1953 UNITED STATES PATENT OFFICE ELECTROMECHANICAL FILTER Application December 14, 1948, Serial No. 65,253

.9 Claims.

This invention relates generally to frequency selective circuit components and more particularly to an electromechanical filter suitable for use at medium and high radio frequencies.

In the prior art electrical filter circuits have been generally used to provide frequency selection in radio and related high frequency applica tions. Mechanical wave filters "have also been used to some extent, particularly at audio frequencies and to some extent at medium frequencies. However, the prior units have not been entirely satisfactory, first because the mechanical structures thereof are diflicult to manufacture in large quantities with therequired tolerances, and second because the output response characteristics of the filters have not completely fulfilled the desired requirements.

It is, therefore, an object .of the present invention to provide an improved compact electromechanical filter structure.

A further object of this invention is to provide an electromechanical filter for providing high selectivity at medium and high frequencies.

Another object of this invention is to provide an electromechanical filter the response of which provides high selectivity without producing undesired ringing in the circuits in which it is used.

still further object of this invention is to provide an electromechanical filter structure including damping means for controlling the response characteristics of the filter.

A feature of this invention is the provision of an electromechanical filter including. a plurality of mechanically coupled vibrating elements with driving means and matching coils all constructed as a compact unit.

Another feature of this invention is the prov..s-on of an electromechanical filter including a plurality of coupled plates the individual characteristics of which are selectedso that the filter has a desired overall response characteristic.

A further feature of this invention is the provision of an electromechanical filter including a plurality of mechanically coupled plates and damping means engaging the plates for controlling the response characteristics of the filter.

Still another feature of this invention is the provision of a filter and damping means therefor including adjustable means for controlling the amount of damping.

A still further feature of this invention is the provision of a filter and a housing therefor adapted to contain liquid damping material surrounding the filter structure.

Further objects, features and advantages will be apparent from a consideration of the following description when taken in connection with the accompanying drawings in which:

Fig. l is a cross-sectional view illustrating one embodiment of the filter structure in accordance with the invention;

Fig. 2 is a fragmentary detail view illustrating the filter structure;

Figs. 3 and 4 are cross-sectional views along the lines of Figs. 33 and i-4 of Fig. 1 respectively;

Fig. 5 is a schematic diagram illustrating the use of the filter in an electronic circuit:

Figs. 6 and 7 illustrate a modified damping structure for the assembly of Fig. 1;

fig. 8 is a perspective View partly in section of a modified filter structure;

Fig. 9 is a cross-sectional view along the lines t% of Fig. 8;

Fig. 10 is a curve chart illustrating the band pass characteristics of the filter;

Fig. 11 illustrates the response characteristics of filters of different constructions;

Fig. 12 illustrates a modified filter structure;

Figs. 13 and i4 illustrate the selectivity of the structure of Fig.12; and

Fig. 15 illustrates still another filter structure.

In practicing the invention there is provided an electromechanical filter comprising a plurality of plates having the same mechanical resonant frequency, mechanically interconnected by fine wires so that vibrations are transmitted from plate to plate. The end plates of the filter are made of magnetostrictive material and are placed in fields produced by permanent magnets and coils. The coils are connected to matching transformers for coupling the same in high impedance circuits. A shielding structure is provided between the driving and pickup coils of the assembly and about the compact filter structure to form a housing for the entire filter assembly. The response characteristics of the filter may be controlled by the selection of the band pass characteristics of the individual sections and/or by damping of the plates and coupling wires. The plates and coupling wires may be clamped by sheets of resilient material or by relatively heavy liquid. Adjustable means are provided for varying the pressure on the dampillg material so that the damping providedthereby may be altered to provide the required respouse characteristics in the filter.

Referring now to the structure of Figs. 1-4 inelusive, the filter structure includes a ladder-like filter unit including plates ID and H, and wires I2 welded to the plates. As best seen in Fig. 2, the wires lap over the edge of the plates and are spot welded thereto to form a single ladder unit. The filter unit is enclosed in a tubular housing Hi made of any suitable insulating material and having a rectangular cross-section as best shown in Figs. 3 and 4. The coils for driving the plates and the matching coils therefor are provided on insulating members l5 in the forms of hollow spools with extensions l6 thereon supporting the driving coils ll. The extensions it with the coils ll thereon are positioned within the form i i and surround the end plates Id of the filter unit. As previously stated, the end plates it of the filter unit are made of magnetostrictive material so that mechanical vibration is produced in response to current in the coils ll. Permanent magnets iii are positioned adjacent the coils to provide a unidirectional biasing field through the magnetostrictive plates H].

For coupling the windings it in high impedance circuits, matching coils are provided having primary windings 2i] and secondary windings 2i wound on the spools or members !5. The secondary winding 2! is connected to the coil I? and the primary winding 2% is connected to pins 22 for connection to external components. The pins 22 are positioned in the circular end plates 23 of the insulating members i5. Supported within the hollow spools of the members 95 are cores 261 for varying the inductance of the wind-- ings 2!] and 2i of the matching coils. The cores 24 include stems 25 which are threaded in the end plates 23.

Although the coils I? have been referred to generally as driving coils, in actual use one of the coils will be a driving coil and the other a pick up coil. The coils will, however, be of identical construction. To clarify further descrip tion of the filter, the letter a will be added to the numerals designating the pick up coil and associated components. Therefore, the pick up coil is Ila, the matching windings are 28a and Zia and the filter plate within the pick up coil is Cylindrical shielding discs 2% are provided about the tubular housing It to prevent direct electromagnetic and electrostatic coupling between the driving and pick up coils and the associated. matching windings. A tubular shield 2? is pro vided about the entire assembly and secured to the end plates 23 to form a compact unitary structure. This is accomplished by rolling a rib 28 near the ends of the shield 21 and rolling the ends over against washers 29.

In order to damp out undesired vibrations in the filter, damping means are provided. These include strips of resilient material 3S positioned about the end plates it for termination of the filter. Sheets of resilient material 3! are positioned on the two sides of the filter unit in engagement with the plates H and the coupling wires l2 to damp out undesired flexural vibrations of the filter unit itself. In order to render manufacture of the filter less critical and to permit accurate adjustment of the response characteristics thereof, adjustable means are provided for varying the pressure exerted by the sheets 3| on the filter components. This is provided by the screws 32 which are adjustably threaded in member 33 connected to the shield ing discs 26. The screws extend through openings 34 in the tubular housing [4 and bear on a stiff plate 35 positioned above the top damping sheet 3|. The plate 35 therefor applies pressure 4, on the damping sheets which may be controlled along the length of the filter by the adjustment of the individual screws 32. Adjustment of the damping pressure provides variation in the response characteristic as will be fully explained.

The electromechanical filter may be used for selecting particular frequencies in various circuit arrangements. One application in which the filter is particularly applicable is in an intermediate frequency amplifier. Fig. 5 illustrates the use in such a circuit with the tube 45 applying signal energy to the filter, and the filter applying the selected signals to the input electrodes of the tube 4|. The tube d0 may be used either as a mixer or an intermediate frequency amplifier and the tube t] in either case would be an intermediate frequency amplifier. The primary matching winding 26 is resonated by a condenser 42 and applies the signal to a secondary winding 2!. The signal is then applied to the driving winding H which produces a field reacting with the field of permanent magnet it to cause movement of the magnetostrictive plate IE3. A condenser 43 is provided for resonating the secondary circuit. Such movement is transmitted in turn through the plates l l to the plate Illa at the other end of the filter with each of the plates acting to further select the frequency to which they are resonant. Movement of the plate Ilia in its magnetic field induces a voltage in the pick up coil ila which is applied through the windings Zla and 29a of the matching transformer to the grid 44 of the tube 4 l. A condenser 35 may be provided for series resonating the coil lid and a condenser 46 may be connected across the winding 26a to resonate the same in a well known manner.

The plates of the filter unit are constructed of such size that the natural frequency of vibration across the width or shortest dimension thereof is at the desired frequency. The driving plates (iii and Illa) must be made of a matnetrostrictive material such as nickel and the intermediate plates should be made of material such that the ratio of modulus of elasticity to density is substantially constant through wide temperature variations. Ailoys oi the Invar type and certain precipitation hardening alloys have been found to be particularly suitable. Such alloys are advantageous as they have s cient magnetostrictive properties that the r nant frequency of individual plates can be brated by driving the plates magnetostrictiveiy. For operation at an intermediate frequency, of about 450 kilocycles per second, the dimensions of the plates are approximately .38?) inch de, .200 inch long .010 inch thick. The coup 21g wires may be made of material such tungsten or molybdenum. Both materials have been used satisfactorily. In a structure as shown, tungsten wire having a diameter of about 6 thousandths of an inch, and molybdenum wire of about 7 thousandths, have been found to be suitable.

The tubular housing i may be made of any suitable insulating material such as Bakelite impregnated paper. The damping material may be in the form of rubber sheets, with rubber of the silicon and neoprene types being preferable. Various plastic materials are also suitable as are strips of felt or rayon flocking. The plate 35 which applies pressure on the damping sheets may be any stiflf material and may be either metal or insulation.

As previously stated, the filter structure may be used as the selective element of an intermeaesaeee diate frequency amplifier. For such. use it. is desiredthat thefilter passa definite band. of frequencies and attenuate all other frequencies. In other words, the passband for. maximumselectivity should be rectangular. with steep, sides and a substantially square top. It has been found that such characteristics canv be. substantially provided by filter structures as illustrated. The band width can be controlled by selecting the impedance of the coupling elements and the plates. Fig. 10 shows curves of such filters with the curve A illustrating the band pass of a lightly damped filter. It will be noted that the band width at the top is about 20 kilocycles and that the band 50 decibels below thetop is only slightly more than 40 kilocyclesg'wide. This will provide very high selectivity being as good or better as, that attainable by the best electrical filter which would be much larger; and more complicated and'expensive.

Although a band pass characteristic which" is substantially rectangular is required for maximum selectivity, it has been found: thatwhen such a filter is used in standard: circuits, undesiredringing or continued oscillations may occur. Tests have shown thatthe ringing response of 'a filter having a rectangular band pass may beas much as one-half of the main transient response and this will produce undesiredoscillations in the circuit. Curve A ofFig. 11 illustrates the response ofsuch a filter-whenshock excited by a pulse such as would-be produced" by noise in a communication circuit: The noise-pulse is represented at it; the main transient response at- H and the ringingresponse-at 12*- and 13'. It isobvious that ringing responses extending v over a long time interval as-indicated by Hand-1.3" will be objectionable as they might paralyze the communications system for this intervalof time. As the ringing responses are frequencymodulated, they can not be balanced-but infrequency modulation detectors. By-rounding thetops of the bandpass of the filters; theratioofringing response to main transient response can be very greatly reduced. Thisis indicated by curve B-of Fig. 11 in which the pulse-l-producesa main transient response 16 and a ringing response; The ringing response is of lower amplitude and of much shorter duration than in curve-A: It has been found that the band pass characteristicof electromechanical filters can= be rounded in two principal ways; First. the various plates and couplings of the sections-of a filter can be selected so that various sections have diiferent' band widths and in this manner the band pass-characteristics can be shaped; Such plate and-coupling constructions which provide differentband widths, will also provide different-impedances'iin the various sections. Thesecondmethodnomprises damping of the-filters-which-lowers-theQ and produces detuning. This-isentirely analo guns to the damping of electricalcircuitsand changes theeifective impedance of the filter sections. Such damping resultsin-phase'shiftof the various frequencieswithin -the-banda It has been found that when the phase-shift ofthe various. frequencies within'the band is-linear with frequency throughout i the: pass band; the ringingresponse is very low. Phaseshift can becontrolled both by damping and by the construction of the various partsof the'filter'unit:

Fig. 12 shows a filter unit in which one section has a different band pass characteristic than the others. The band passof a late and associated coupling wires is proportionalitothe ratioof; the

impedance of the wires to. the. impedance of'the plate. The impedance of the plates and wires is in, turn proportional to the. cross sectional area thereof. In Fig. 12 the coupling. wires 12. may all be identical and the plate Ila has a greater cross sectional area than that of the plates ll so that the band pass of the section. including the plate I la will be narrower thanthat of the other sections. This is. illustrated. in Fig. 13. in which the curve indicates the bandpass characteristic of any of the plates l I andcurve Bl. indicates the band pass characteristic of the plate Ha. When combined in a filter as" illustrated in. Fig. 12; the overall response is as indicated .in Fig. 14'. To provide such a response the band pass of the sectionincluding section Ha must be. centered with respect to the band pass of the other sections. This is'accomplished by constructing the plate Ha so that the lower cut off frequency f1 differs from the lower cut ofi frequency 11 of the plate I! by one-half the difference in the band widths of the different sections.

The rounded bandpass characteristic as indicated'by the curve of Fig. l4will have a low ratio of ringing response to main transient response. A very small amount of damping; will then produce asmoothly rounded curve as indicated by the dotted lines. Various otherconstructions can be used to shape the bandpasscharacteristics. One example is illustrated in Fig. 15 in'which two narrow band pass sections I I6 are-combined with twowider band pass sections ll. Other combinations can be used with the sections having different band widthsto provide any degree of shaping required. A simple arrangement as-illustrated in Figs. l2, l3 and'l l provides very desirable results however.

As previously stated rounding of'theband pass characteristic can be obtained by simple damping of the plates and coupling wires; Curve B of Fig. 10 illustrates the'pass band of a filter damped to produce minimum ringing response. In tests made, the minimum ringing response may be reduced to as little as of the maintransient response. It is to be noted that curve B in other respects is very similar to curve A and'the sides are sufl'lciently steep. The damping does, of

course, increase the insertion-loss of the filter.

somewhat. Curve C of Fig-urelO shows the filter still further damped so that the top is quite smoothly rounded. However, such heavy damp? ing results in large insertion losses so that the ratio of the ringing response to'the maintransient response may actually increase with increased damping. Also the loss of selectivity because of increased damping increases the duration of the main transient response. Curve C indicates a filter in which the ringing response is of the order of of the main transientresponse. It is to be pointed-out that filters damped-to a more or less extent than that indicated by'curve B will give satisfactory results. It has been found that when the ringing response is of the order of f., or less of the main transient'response, the ringing has no serious consequences; The amount of damping produced by damping sheets; as shown in Figs. 1-4; depends upon the area of contact between the damping sheets and the plates and the pressure exerted by the damping sheets; In a structureas shown in Figs. 1-4, the pressure can be varied bythe screws 32 and can; be adjusted alongv the length of the filter to provide the results.desired.

Iii Figs. 6 and .7 thereisillustrated a-modifiedstructure whichv maybe'usedjfor the lower;

7. damping sheet. In these figures, the sheet is in the form of a molded rubber pad 50 havin raised ridges which engage the coupling wires !2. This provides additional engagement between the damping material and the coupling wires to provide increased damping. The top damping sheet may be flat, as shown in Figs. 1-4.

In Figs. 8 and 9 there is illustrated a still further modified structure. In this structure the filter unit including the plates and coupling wires may be identical to that of Figs. 1-4. However,

the filter unit is supported in a molded channel member til which takes the place of the tubular housing It of Fig. 1. The drive coil l! and pick up coil We. are supported on the ends of the channel and the permanent magnets is may be supported directly on the coils. This structure is adapted to be used with liquid damping with the liquid damping material 61 being contained in the channel of the member (ill. A closure plate 52 and a combined supporting and shielding structure t3 are provided. being secured to the channel Si? by rivets (54. The shielding structure 63 includes a central portion extending along the channel and end portions 65 extending perpendicular to the filter unit and forming shields between the matching windings 2% 2i and 26a, 25a. These portions are shaped to cover substantially the entire cross-sectional area of the outer shield 22?. The matching windings may be mounted on the insulating spools 66 which are generally equivalent to the members 15 of Fig. l, but are of such construction that they can be secured to the end portions 55. The end portions 65 may have openings 6'! therein into which the tubular portion 58 of the members $36 extend for securing the members to the shield.

The outer shield 21 may be identical to that of Fig. 1, and forms a housing for the entire filter structure. The insulating end members 85 are secured in the tubular housing by the rolled ribs and the turned over ends which bear against the washers 29 in exactly the same manner as in Fig. 1.

When using liquid damping, as illustrated in Figs. 8 and 9, various materials having relatively high viscosity such as grease, silicon compound and polybutene may be used. To provide a large amount of damping, materials that introduce large losses should be used. It is also desirable that the characteristics of the damping material remain constant through a Wide temperature range so that the filter will not be affected by changes in temperature. The amount of damping depends upon the contact area, as with solid damping, and also upon the characteristics of the damping liquid. Therefore, by proper selection of damping materials and by controlling the amount of material used, the damping can be controlled. Curves similar to those illustrated in Fig. were obtained using liquid material for damping to indicate that the use of liquid and solid damping materials are equivalent.

It is seen from the above that there is provided a filter structure which is suitable for use as a frequency selective element and which is of very compact construction. In the units disclosed, the outer shielding tube or housing is A of an inch in diameter and about 3 inches long. Electrical filters having highly selective characteristics would require more than ten times this amount of space and would be much more expensive. The construction of the filters are such that they can be produced in large quantities by production line methods. The band pass 8 characteristics can be shaped to reduce the ringing response without introducing material losses. Control of the band pass characteristics by damping, which is possible in the structures shown, makes it possible to control the ringing response so that it is held within satisfactory limits and permits adjustments after the unit is assembled.

While certain embodiments of the invention have been described which are illustrative thereof, it is obvious that various changes and modifications can be made therein without departing from the intended scope of the invention as defined in the appended claims.

We claim:

1. An electromechanical filter comprising a plurality of sections each including a flat plate and means connected to at least one edge of the plate thereof for mechanically coupling said plate to at least one other plate, said plates transmitting vibrations therethrough and said coupling means applying said vibrations from one plate to the next, one of said sections including a plate of diiferent configuration than the plate of another section so that the band pass of said one section is different than the band pass of said other section.

2. An electromechanical filter comprising a plurality of sections each including a thin flat plate and wires connected to the edge of said plate for transmitting motion between said plate and at least one other plate, said plates transmitting vibrations therethrough from one edge to another edge, said wires connected to all said plates being substantially identical and said plates being of such configuration that the mechanical resonant frequencies thereof are substantially the same and the impedances thereof differ so that the bands passed by the various sections differ.

3. An electromechanical filter comprising a plurality of sections each including a flat plate and coupling means connected to an edge of said plate so that each of said sections forms a band pass filter for selectively transmitting vibrations therethrough, the plate and coupling means of one section being so constructed that the band pass of said one section is of less width than that of the other sections and the center frequency of the band pass of all said sections is substantially the same, said sections having such impedance that the phase shift applied thereby to vibrations transmitted therethrough varies substantially linearly with frequency within the band pass of the filter.

4. An electromechanical filter including in combination, a plurality of thin plates having substantially parallel edges, means connected to said edges of said plates for transmitting motion from one plate to another, said plates being of such configurations that the mechanical resonant frequencies thereof are substantially the same and the impedences thereof are different so that the band pass of the various plates are different, and damping means engaging said plates for altering the effective impedence thereof so that the phase shift of vibrations transmitted through said filter varies substantially linearly with frequency.

5. An electromechanical filter including including in combination, a plurality of thin plates having substantially parallel edges, damping means engaging said plates for preventing flexural vibrations thereof, and means connected to said edges of said plates for transmitting motion from one plate to another, said plates having substantially the same lengths between said parallel edges so that the mechanical resonant frequencies thereof are substantially the same, and having different cross sections so that the impedances thereof and the band pass provided thereby are difierent.

6. An electromechanical filter including in combination, a plurality of thin flat plates having substantially parallel edges, means supporting said plates for vibration so that vibrations are transmitted therethrough from one of said edges to the opposite edge, and mechanical connecting means coupling said plates for transmitting vibrations from one plate to another, said connecting means being secured to each plate along at least one edge thereof and having relatively small cross section as compared to the cross section of said plates, said supporting means including damping means engaging said plates for altering the eifective impedance thereof, said plates having such dimensions that the resulting effective impedance thereof including the effect thereon of said damping means produces phase shift of the vibrations transmitted through said plates which varies substantially linearly with frequency, said damping means including a sheet of resilient material having projecting ribs thereon positioned to engage said mechanical connecting means in spaces between said plates.

7. An electromechanical filter in accordance with claim 4 in which said damping means is a liquid having relatively high viscosity.

8. An electromechanical filter in accordance with claim 4 in which said damping means includes sheets of resilient material engaging said plates and said connecting means, and means for adjusting the pressure exerted by said sheets.

i0 9. An electromechanical filter structure including in combination, a mechanical filter unit having a plurality of fiat rectangular plates adapted to transmit vibrations therethrough from one edge thereof to the opposite parallel edge, and mechanical means secured to at least one edge of each of said plates and coupling said plates in a series, a driving coil about the plate at one end of said unit and a pickup coil about the plate at the other end of said unit, an elongated conducting tubular housing about said unit, end members closing the ends of said tubular housing providing a completely enclosed structure, a pair of conducting discs positioned between said driving coil and said pickup coil having openings for receiving said filter unt therethrough and edges engagng said conducting housing, and damping means engaging the sides of said plates for preventing fiexural vibrations thereof, said conducting discs supporting said filter unit and said damping means from said housing, said conducting disc and said conducting housing preventing direct electromagnetic and electrostatic coupling between said driving coil and said pickup coil.

MYRON L. ANTHONY.

ROBERT M. VIRKUS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,678,116 Harrison July 24, 1928 1,892,554 Kellogg Dec. 27, 1932 2,318,417 Phelps May 4, 1943 2,445,762 Chanal July 27, 1948 2,455,740 Curtis Dec. 7, 1948 2,501,488 Adler Mar. 21, 1950 

